[Intestinal failure and transplantation in microvillous inclusion disease]

MYO5B-associated diseases: Novel liver-related variants and genotype-phenotype correlation

BACKGROUND & AIMS

Biallelic pathogenic variants in MYO5B cause microvillus inclusion disease (MVID), or familial intrahepatic cholestasis (FIC). The reported FIC patients are scarce and so the genotype-phenotype correlation has not been fully characterised. This study aimed to report more MYO5B-associated FIC patients and correlate genotypes to phenotypes in more detail.

METHODS

The phenotype and genetic data of 12 newly diagnosed MYO5B-associated (including 11 FIC) patients, as well as 118 previously reported patients with available genotypes, were summarised. Only patients with biallelic MYO5B variants were enrolled. Nonsense, frameshift, canonical splice sites, initiation codon loss, and single exon or multiexon deletion were defined as null MYO5B variants.

RESULTS

Phenotypically, 50 were isolated MVID, 47 involved both liver and intestine (combined), and 33 were isolated FIC (9 persistent, 15 recurrent, 3 transient, and 6 un-sub-classified) patients. The severity of intestinal manifestation was positively correlated to an increased number of null variants (ρ = 0.299, P = .001). All FIC patients carried at least one non-null variant, and the severity of cholestasis was correlated to the presence of a null variant (ρ = 0.420, P = .029). The proportion of FIC patients (16/29, 55%) harbouring missense/in-frame variants affecting the non-motor regions of MYO5B was significantly higher than that of MVID (3/25, 12%, P = .001) and combined patients (3/31, 10%, P = .000). 10 of the 29 FIC patients harboured missense/in-frame variants at the IQ motifs comparing to none in the 56 MVID and combined patients (P = .000).

CONCLUSIONS

The phenotype of MYO5B deficiency was associated with MYO5B genotypes, the nullity or the domain affected.

Congenital Diarrhea and Enteropathies in Infants: Approach to Diagnosis

Congenital diarrhea and enteropathies (CODEs) are monogenic disorders causing early onset of intractable diarrhea. Their diagnosis and management are challenging. With the availability of commercial next generation genetic testing, we are now better able to classify and manage these disorders. The authors present their experience with 4 cases. Two patients had congenital tufting enteropathy (CTE) and 1 case each of microvillous inclusion disease (MVID) and trichohepatoenteric syndrome (THES). Age at onset varied from 3 to 38 d of life. Light microscopy and electron microscopy of duodenal and rectal endoscopic biopsies were consistent with the diagnosis. Genetic evaluation was possible in 3 cases indicating causative mutations. Two children (CTE and MVID) were alive at last follow-up. The authors suggest a stepwise approach to the diagnosis and management of these disorders in the Indian context.

Experimental Studies on the Impact of the Projected Ocean Acidification on Fish Survival, Health, Growth, and Meat Quality; Black Sea Bream (Acanthopagrus schlegelii), Physiological and Histological Studies

Simple Summary

This study’s data suggest that under the projected scenarios of ocean acidification by 2100 and beyond, significant negative impacts on growth, health, and meat quality are expected, particularly on black sea bream, and will be susceptible to the scientifically approved fish having a weaker resistance to diseases and environmental changes if CO₂ emissions in the atmosphere are not curbed. Knowing the expected consequences, mitigation measures are urgently needed.

Abstract

Acidification (OA), a global threat to the world’s oceans, is projected to significantly grow if CO₂ continues to be emitted into the atmosphere at high levels. This will result in a slight decrease in pH. Since the latter is a logarithmic scale of acidity, the higher acidic seawater is expected to have a tremendous impact on marine living resources in the long-term. An 8-week laboratory experiment was designed to assess the impact of the projected pH in 2100 and beyond on fish survival, health, growth, and fish meat quality. Two projected scenarios were simulated with the control treatment, in triplicates. The control treatment had a pH of 8.10, corresponding to a pCO₂ of 321.37 ± 11.48 µatm. The two projected scenarios, named Predict_A and Predict_B, had pH values of 7.80-pCO₂ = 749.12 ± 27.03 and 7.40-pCO₂ = 321.37 ± 11.48 µatm, respectively. The experiment was preceded by 2 weeks of acclimation. After the acclimation, 20 juvenile black sea breams (Acanthopagrus schlegelii) of 2.72 ± 0.01 g were used per tank. This species has been selected mainly due to its very high resistance to diseases and environmental changes, assuming that a weaker fish resistance will also be susceptibly affected. In all tanks, the fish were fed with the same commercial diet. The seawater’s physicochemical parameters were measured daily. Fish samples were subjected to physiological, histological, and biochemical analyses. Fish growth, feeding efficiency, protein efficiency ratio, and crude protein content were significantly decreased with a lower pH. Scanning electron microscopy revealed multiple atrophies of microvilli throughout the small intestine’s brush border in samples from Predict_A and Predict_B. This significantly reduced nutrient absorption, resulting in significantly lower feed efficiency, lower fish growth, and lower meat quality. As a result of an elevated pCO₂ in seawater, the fish eat more than normal but grow less than normal. Liver observation showed blood congestion, hemorrhage, necrosis, vacuolation of hepatocytes, and an increased number of Kupffer cells, which characterize liver damage. Transmission electron microscopy revealed an elongated and angular shape of the mitochondrion in the liver cell, with an abundance of peroxisomes, symptomatic of metabolic acidosis.

Congenital Diarrhea and Cholestatic Liver Disease: Phenotypic Spectrum Associated with MYO5B Mutations

Myosin Vb (MYO5B) is a motor protein that facilitates protein trafficking and recycling in polarized cells by RAB11- and RAB8-dependent mechanisms. Biallelic MYO5B mutations are identified in the majority of patients with microvillus inclusion disease (MVID). MVID is an intractable diarrhea of infantile onset with characteristic histopathologic findings that requires life-long parenteral nutrition or intestinal transplantation. A large number of such patients eventually develop cholestatic liver disease. Bi-allelic MYO5B mutations are also identified in a subset of patients with predominant early-onset cholestatic liver disease. We present here the compilation of 114 patients with disease-causing MYO5B genotypes, including 44 novel patients as well as 35 novel MYO5B mutations, and an analysis of MYO5B mutations with regard to functional consequences. Our data support the concept that (1) a complete lack of MYO5B protein or early MYO5B truncation causes predominant intestinal disease (MYO5B-MVID), (2) the expression of full-length mutant MYO5B proteins with residual function causes predominant cholestatic liver disease (MYO5B-PFIC), and (3) the expression of mutant MYO5B proteins without residual function causes both intestinal and hepatic disease (MYO5B-MIXED). Genotype-phenotype data are deposited in the existing open MYO5B database in order to improve disease diagnosis, prognosis, and genetic counseling.

A Molecular Mechanism Underlying Genotype-Specific Intrahepatic Cholestasis Resulting From MYO5B Mutations

BACKGROUND AND AIMS

Progressive familial intrahepatic cholestasis (PFIC) 6 has been associated with missense but not biallelic nonsense or frameshift mutations in MYO5B, encoding the motor protein myosin Vb (myoVb). This genotype-phenotype correlation and the mechanism through which MYO5B mutations give rise to PFIC are not understood. The aim of this study was to determine whether the loss of myoVb or expression of patient-specific myoVb mutants can be causally related to defects in canalicular protein localization and, if so, through which mechanism.

APPROACH AND RESULTS

We demonstrate that the cholestasis-associated substitution of the proline at amino acid position 600 in the myoVb protein to a leucine (P660L) caused the intracellular accumulation of bile canalicular proteins in vesicular compartments. Remarkably, the knockout of MYO5B in vitro and in vivo produced no canalicular localization defects. In contrast, the expression of myoVb mutants consisting of only the tail domain phenocopied the effects of the Myo5b-P660L mutation. Using additional myoVb and rab11a mutants, we demonstrate that motor domain-deficient myoVb inhibited the formation of specialized apical recycling endosomes and that its disrupting effect on the localization of canalicular proteins was dependent on its interaction with active rab11a and occurred at the trans-Golgi Network/recycling endosome interface.

CONCLUSIONS

Our results reveal a mechanism through which MYO5B motor domain mutations can cause the mislocalization of canalicular proteins in hepatocytes which, unexpectedly, does not involve myoVb loss-of-function but, as we propose, a rab11a-mediated gain-of-toxic function. The results explain why biallelic MYO5B mutations that affect the motor domain but not those that eliminate myoVb expression are associated with PFIC6.